The present invention relates to radio engineering and is applicable to antenna feeder devices, mainly to compact super-broadband antennas.
A conventional spiral antenna is made by conductors arranged in a single plane and formed into a bifilar rectangular spiral with turns directed opposite to each other (1).
The spiral antenna exhibits a relatively enhanced broadbanding as compared to the other types of antennas, such as dipole antennas, folded antennas, Y-antennas, rhombic antennas, etc.
However, to further enhance the broadbanding, the bifilar helix must be quite large, especially in cases when it is required to provide operation in the low-frequency range.
Another conventional antenna comprises antenna elements arranged in a single plane and coupled opposite to each other (2).
In this prior art, the antenna elements are plates in the shape of isosceles triangles with oppositely directed vertices, the opposite sides of the triangles being parallel to each other. The advantage of this antenna is that it is constructed on the self-complementarity principle according to which the shape and size of the metallic portion correspond and are equal to those of the slot portion complementing the metallic portion in the plane. Such infinite structure exhibits a purely active, frequency-independent input resistance, which improves its matching within a broad range of frequencies.
However, this antenna suffers a reduced broadbanding by input resistance due to finiteness of its geometrical dimensions.
Most closely approaching the present invention is an antenna comprising a spiral antenna made by conductors arranged in a single plane and formed into a bifilar helix, turns of the helix being directed opposite to each other, two antenna elements disposed in the same plane and oppositely coupled to the conductors at outer turns of both spiral paths of the bifilar helix, respectively (3).
In this system, the antenna elements form a half-wave dipole (or monopole) antenna with arms made by two pins. The above antenna system overcomes, to a certain extent, the problems of conventional antennas. The spiral antenna operates in the high-frequency range, while the boundary of the low-frequency range depends on the antenna""s diameter and is of the order of 0.5xcex, where xcex is the working wavelength. Beginning from these frequencies, the half-wave dipole antenna is brought into operation. The half-wave dipole antenna may be coupled to the spiral antenna either at outer or inner termination points.
The antenna system in accordance with the most pertinent prior art suffers the following deficiencies:
it has considerable geometrical dimensions because the size of the spiral should be no less than 0.5xcex, and the size of the dipole antenna should be 0.5xcexmax;
its broadbanding is insufficient because the half-wave dipole antenna is a narrow-band device, and the input resistance varies as a function of frequency at the connection points of the dipole arms, this significantly affecting the broadbanding of the system;
the galvanic coupling of two antenna systems with different resistances impairs the quality of matching.
The object of the present invention is to improve performance and extend the stock of employed technical means.
The present invention provides an antenna that exhibits an enhanced broadbanding and improved standing wave ratio (SWR), is simple in construction while maintaining a small size.
The object of the present invention can be attained in a conventional antenna comprising a spiral antenna made by conductors disposed in a single plane and formed into a bifilar helix, turns of the bifilar helix being directed opposite to each other, two antenna elements arranged in the same plane and coupled, oppositely to each other, to termination points of the conductors at outer turns of the bifilar helix, respectively, wherein in accordance with the present invention, the bifilar helix is a rectangular spiral made by line segments with right angles of the turns, each of the antenna elements forming an isosceles trapezoid and coupled to a termination point of a conductor at a vertex of the smaller base of the isosceles trapezoid, the bases of the isosceles trapezoids being parallel to the line segments of the bifilar helix.
In further embodiments of the antenna in accordance of the invention it may be provided that
the line segments of the bifilar helix are straight;
the conductors are formed into a square-shaped bifilar spiral;
distances between opposite vertices of the large bases of the isosceles trapezoids of the antenna elements are equal to each other and to a distance between all adjacent vertices of the large bases;
sizes of spacings between the conductors of the bifilar helix are equal to a thickness of the conductors;
length L of the smaller base of the isosceles trapezoid is L=l+2xcex4, where l is the length of the straight-line segment of the turn of the bifilar helix, directed to the base of the isosceles trapezoid, and xcex4 is the size of the spacing between the turns of the bifilar helix;
the antenna element is a solid plate;
the antenna element is a zigzag thread having bending angles which correspond to the shape of an isosceles trapezoid, so as zigzag parts of the zigzag thread coincide with the lateral sides of the isosceles trapezoid, and the connecting zigzag parts of the zigzag thread are parallel to the bases of the isosceles trapezoid;
sizes of the spacings between the conductors of the bifilar helix are equal to sizes of spacings between the parts of the zigzag thread which are parallel to the bases of the isosceles trapezoid;
the zigzag thread of the antenna elements forms a meander along its longitudinal axis;
the zigzag thread of the antenna elements forms, along its longitudinal axis, a constant pitch structure which is defined, within the constant pitches, by a pseudo-random sequence of digits 0 and 1 with the same average frequency of occurrence of the digits;
each of the conductors forms a meander along its longitudinal axis;
each of the conductors of the bifilar helix forms, along its longitudinal axis, a constant pitch structure which is defined, within the constant pitches, by a pseudo-random sequence of digits 0 and 1 with the same average frequency of occurrence of the digits;
the conductors and the antenna elements have a high resistivity.
The above object of the present invention has been attained owing to forming the antenna into a bifilar rectangular spiral and using the antenna elements in the shape of an isosceles trapezoid. The antenna system (AS), in general, is constructed on the self-complementarity principle; it includes a bifilar rectangular Archimedes spiral; extensions of the bifilar helix are plates having a width linearly increasing with a distance from the center of the helix, or a conductive zigzag thread which fills the area of the plates. Broadbanding of the AS may be further enhanced by making all of the conductors meander-shaped and of a high-resistivity material.
FIG. 1 shows an embodiment of an antenna in accordance with the present invention with antenna elements made by plates in the shape of isosceles trapezoids;
FIG. 2 shows an embodiment of an antenna in accordance with the present invention, formed by a bifilar rectangular Archimedes spiral continued by a zigzag thread having a width linearly increasing with a distance from the center of the spiral;
FIG. 3 shows an embodiment of an antenna in accordance with the present invention, in which all of the conductors and the zigzag threads of the antenna elements form meanders;
FIG. 4 shows an embodiment of an antenna in accordance with the present invention, in which all of the conductors and the zigzag threads of the antenna elements form a non-periodic constant pitch meander structure, with periods in the structure being defined by a pseudo-random sequence of digits 0 and 1 with the same average frequency of occurrence of the digits,
FIG. 5 is a plot of the standing wave ratio (SWR) adjusted to the characteristic impedance of 75 Ohm.